@hsc: Thanks for your report.
I have sent you an email with a debug version of WinSCP to the address you have used to register on this forum.

<Try to describe precise steps that lead to the problem (where do you click, what keys do you press, what do you see, etc.)>
<If relevant, consider attaching a session log file or a screenshot)>

WinSCP 6.3.3

Error message:
External exception C0000006

Stack trace:
(00E21123)
(0008239F) ntdll.dll
(0006FEFA) ntdll.dll.KiUserExceptionDispatcher
(00C081F5)
(00C082D4)
(00C085AA)
(00C0860C)
(0002F201)
(003C2ECB)
(0002C8E4)
(00404951)
(00409495)
(003C39A4)

Status: unknown_rel_addr

I know how to reproduce the problem or the problem happens frequently enough. I wish to be contacted by the WinSCP team to help resolving the problem.